The posterior crescent sign on MRI and MR arthrography: is it a marker of hip dysplasia and instability?

To evaluate the prevalence of the 'posterior crescent sign' in symptomatic patients referred for MRI/MR arthrogram of the hip and identify any correlation with imaging features of joint pathology. Retrospective imaging assessment of a cohort of 1462 hips, from 1380 included MR examinations (82 bilateral) retrieved from a search of all examinations in patients 16-50 years old from June 2018 to June 2021, with median age 45.8 years (range 17.8-50.0) and 936 hips (64%) in women. Radiographic and MR findings related to hip dysplasia, femoroacetabular impingement and osteoarthritis were assessed. Fifty-one hips (3.5%) were positive for the posterior crescent sign, median age of 45.8 years (range 17.8-50.0) and 29 (58%) in women. Radiographic findings included the following: mean lateral centre edge angle (LCEA) 22.2° (± 7.8°) with LCEA < 20° in 15 (31%) and LCEA 20-25° in 17 (35%) and mean acetabular index (AI) of 13.1° (± 5.8°) with AI > 13° in 22 (45%). MR findings included the following: mean anterior acetabular sector angle (AASA) 54.3° (± 9.8°), mean posterior acetabular sector angle (PASA) 92.7° (± 7.0°), labral tear at 3-4 o'clock in 20 (39%), high-grade acetabular chondral loss in 42 (83%) and ligamentum teres abnormality in 20 (39%). The posterior crescent sign occurs in 3.5% of symptomatic young and middle-aged adults on MR. It is associated with overt and borderline hip dysplasia and other findings of hip instability. It is also associated with osteoarthritis in some cases and should be interpreted with caution in these patients.